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IBS has many sub-types: IBS-C (constipation predominant), IBS-D (diarrhea predominant), IBS-alternating (both constipation and diarrhea), IBS post-infectious (IBS caused as a result of gastrointestinal infection), and Leaky Gas (not officially recognised as an IBS sub-type).


IBS is probably caused by a bacterial overgrowth coupled with carbohydrate malabsorption. If carbohydrates aren't fully digested in the small intestine, these foods feed bacteria, which in turn produces gas.

IBS is a vicious cycle where carbohydrate malabsorption means that bacteria eat the food meant for us. This in turn results in greater bacteria populations as this cycle continues.

Not a lot is known about bacteria in the colon as it's hard to do testing there.

Generally, it's believed bad bacteria feed on refined starch and sugar and good bacteria feed on vegetables.

Bad bacteria could also mean good bacteria living in the small intestine, where they don't belong. Bacteria normally reside in the colon, the large intestine.

Meat and fish are digested in the stomach. Carbohydrates are all plant foods and include starch and fibre. All carbohydrates are made up of glucose, but the glucose chains in vegetables are difficult to break apart. This is called fibre. Some calories can be obtained from fibre in the colon and so some fibre can be digested and converted to calories. Bacteria in the colon release enzymes that can partially digest the soluble fibre in vegetables. Because of this, soluble fibre is also fermentable to a certain extent.

Starch is digested in the small intestine by enzymes lining the small intestine. For reasons not known, not all people can readily digest starch. This could be due to a lack of enzymes due to age or alcohol abuse causing damage to the intestinal lining.

FODMAPs are fermentable carbohydrates. The complex sugars in these foods are poorly absorbed in the small intestine. These poorly absorbed substrates provide food for bacteria, which in turn causes gas.

FODMAPS = Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols.

FODMAPs cause an osmotic reaction in the colon, which can lead to diarrhea and or loose motions / incomplete evacuation. Fructose is a very common FODMAP sugar intolerance. Most people can't digest too much fructose at once, and people with fructose intolerance can't digest much fructose at all. Fructose isn't taken up by the GLUT transporters in the small intestine that help to carry fructose to our liver and cells. This fructose travels through the small intestine and colon, unabsorbed. Fructose is a polar molecule with an affinity for water. Fructose draws in water from the colon, because there is mutual attraction between water and the fructose molecule. This can lead to diarrhea. The unabsorbed fructose also provides food for bacteria, which causes the production of gasses, generally hydrogen and methane. These gasses can produce either diarrhea or constipation in some people, depending on their genetic make-up.

Most people with IBS find that a low FODMAPs diet eases symptoms.

Gas = constipation.

Dr Pimental did a study on this, showing that methane causes constipation. Other studies showed that people who are predominantly hydrogen producers tend towards diarrhea. Sometimes, too, gas can physically block the passage of stool, causing incomplete evacuation.

Leaky Gas

Meat and seafood doesn't cause gas, unless you swallow a lot of air as you're eating it. Meat and fish are mostly digested in the stomach. Too much animal protein can be constipating, exacerbating IBS symptoms due to the retention of gas-producing carbohydrate as well. This is why constipation is much more distressing for those with IBS than the normal population. The constipation means that all stool (meat as well as malabsorbed carbohydrate) is retained. The malabsorbed carbohydrate will gradually work its way down the colon during the course of the day. The pressure of this causes a build up of gas in the colon. For some people, this describes "Leaky Gas", the IBS subtype which has elements of both IBS-C and IBS-D. Incomplete evacuation and loose stools are a typical symptom of Leaky Gas.

Fibre has to be gradually introduced

The fermenting of fibre is due to the action of bacterial enzymes. Bacterial populations adapt according to your diet. Meat eaters have high levels of certain bacteria; the same with refined starch and sugar. Certain bacteria favour these foods over others. The good bacteria favour fibre and largely ignore sugar and refined starch. We need to introduce fibre gradually if we're not used to it, so that populations of vegetable-feeding bacteria can be given time to grow. If there isn't enough bacteria to produce the enzymes, then fibre won't be properly digested in the colon. It is believed that the "wrong" type of bacteria then ferment the undigested fibre.

Vegetables tend to be either high in starch or fibre. Only experimentation will tell you which of these is better absorbed for you.


Our digestive systems were designed to eat meat and vegetables, and a small amount of fruits when in season. Wheat and rice are grass seeds. Our guts didn't evolve to eat grass seeds, which are largely starch. If we don't produce enough brush border enzymes, then wheat and rice will travel through undigested, feeding bacteria, which causes gas, and in the case of wheat, an osmotic effect in the colon due to its fructans (which are long chains of fructose).


Wheat is highly immunogenic. The innate immune system can mount a defence against amino acid patterns in wheat, which it sees as a threat. These patterns are called PAMPS (pathogen associated molecular patterns). Our immune system is primed to recognise these amino acid patterns as invaders and will try to attack them. This causes inflammation in the colon due to the release of inflammatory chemicals such as cytokines. The Paleo theory believes that wheat is a primary cause of all autoimmune disease, where the immune system attacks its own body.


All wholegrains contain lectins that inhibit digestion, causing constipation. White rice, although it doesn't contain many lectins, is highly constipating. Which leads to a general IBS rule:

If it's not a low FODMAP, low STARCH vegetable - it's constipating

Fruits can be difficult to digest due to their fructose content. Fructans, which are chains of fructose (e.g. wheat) can be even more difficult to digest.

Meds don't help IBS as they make the colon more lazy and can mask the foods that are causing you harm, making IBS worse in the long run.

Diarrhea can be a protective mechanism that the colon uses to protect against an imagined pathogen. Chronic diarrhea might be a symptom of a low level autoimmune disease. If so, continued use of meds will make things worse. The offending food is still being eaten, while meds are taken to counteract the diarrhea. In this case, IBS becomes an inflammatory disorder. It would be better to stop eating that food completely, and to revert to a bland diet that doesn't cause diarrhea, and then gradually introduce the suspect food once symptoms subside.

Often the offending food is harmless by ordinary standards. Sometimes the problem food is simply a higher intake of fibre. People trying to introduce more vegetables shouldn't persist if their new diet is giving them diarrhea. This could indicate that they don't as yet have the enzymes to digest the carbohydrates. Sometimes this takes time, as the bacteria providing the enzymes need time to adapt. You might have low populations of good bacteria that provide these enzymes. It would be better to revert to your old diet immediately until the diarrhea stops. Then very gradually introduce a small amount of a new vegetable. Some people may need to boil low FODMAP vegetables for a very long time at first until their digestive system gets used to the extra fibre.
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